If the answer is nothing, that's a bit of a minus for me. I don't want to worry about whether my skill use is legitimate or whether I'm breaking the game with XP gain. Getting XP in games can be like an unwanted way of shifting the difficulty bar.
Personally, I'd suggest a set XP gain per mission for all party members. Like this there's no need to worry about whether a particular strategy is breaking the game via XP gain and there's no need to worry about your character's being under leveled due to usage of a certain strategy either. On top of all that, all characters are equal levels (without any hassle) allowing for the full variety in party members as opposed to having a few characters virtually removed as effective party members until you've leveled them.
Of course, some might prefer how it is now, but I suspect most players who are in it for the tactics element would prefer it my way, basically (not to say that it would be perfect for them, but it would probably generally be an improvement for those types of player).
I might mod something like this in myself, but I don't know what appropriate levels would be for each battle.
